Ethylene from CO2

Anthony King

Scientists in the US have developed a copper catalyst to more efficiently convert CO2 into ethylene (C2H4).

Presently, 158m t/year of ethylene are made from hydrocarbons such as natural gas, and mostly turned into polyethylene plastic. Developing efficient electrocatalysts for reducing CO2 to value-added fuels and chemicals provides a feasible path for renewable energy storage. It could turn CO2 in air into useful hydrocarbons and reduce rising CO2 emissions from human activities.

Copper is the only electrocatalytic material that efficiently converts CO2 into hydrocarbons. However, the process is too slow and inefficient for industrial production, and also produces undesirable hydrogen and methane.

The researchers in California developed copper wires with a specially shaped surface to catalyse the greenhouse gas into ethylene conversion (Nature Catalysis, 2020, 126, 4173). The nanowires were designed with highly active ‘steps,’ similar to a set of atomic scale stairs.

‘We describe ‘a unique stepped surface that supresses both hydrogen evolution and methane production under CO2 reducing conditions in neutral solution, pH 6.8,’ explains lead author Chungseok Choi at the University of California, Los Angeles (UCLA). Selectivity was achieved not by promoting ethylene production, but by simultaneously dampening competitive hydrogen and methane generation.

‘Compared with previous reports, our catalyst showed about 10% higher efficiency in ethylene production,’ says Choi. The stepped surface remained stable for over 200h.

To be cost competitive, an estimated 65% full cell efficiency is necessary for ethylene production from CO2 with 500mA/cm2 in a full flow cell setting (Science, 2019, 364, eaav3506).

‘This translates to about 80% efficiency in half cell,’ notes Choi. ‘Our result is close to the target at approximately 77% at pH 6.8.’

Commenting on current catalyst technology, materials engineer G. Tayhas Palmore at Brown University, Rhode Island, notes: ‘Methods of preparing CO2 electroreduction catalysts are cost prohibitive [with multiple labour-intensive steps for preparation that are difficult to automate], do not have viable approaches to regeneration and/or recovery, and are not amenable to production at large scale.’

Her group recently reported on the preparation of a copper catalyst with a high density of defect sites, which promoted the adsorption of carbon intermediates and carbon-to-carbon coupling reactions (Nature Communications, doi:10.1038/s41467-020-16998-9). This resulted in high selectivity towards ethylene. Faradaic efficiencies of up to 72% were achieved in converting CO2 to C2+ products.

Other methods of catalyst preparation – such as nanoparticles, nanowires, alloys – often require synthesis, purification, and immobilisation onto carbon supports/current collectors, notes Palmore. ‘Our method eliminates these steps and issues.’